1. Field of the Invention
The present invention relates to a sheet transport apparatus that transports a sheet and to an image forming apparatus that has such a sheet transport apparatus in an apparatus body thereof and forms an image on a sheet transported from the sheet transport apparatus.
2. Description of the Related Art
Conventionally, a sheet handling apparatus that handles a sheet, such as an image forming apparatus that forms an image on a sheet, a sheet processing apparatus that processes the sheet and the like, has a sheet transport apparatus that transports a sheet. Examples of an image forming apparatus include a copying machine, a printer, a facsimile and a combined machine thereof. Examples of a sheet processing apparatus include a sheet binding device that binds a sheet bundle, a punching device that punches a sheet and a folding device that folds a sheet bundle.
Conventionally, in the sheet transport apparatus, when a roller transports a sheet from a sheet holding portion such as a cassette, the roller may sometimes slip on the sheet. When sheets are continuously discharged, the sheet transport apparatus cannot therefore sometimes maintain regular intervals between the sheets. The slippage is caused by a variation of the friction coefficient of the roller due to aging, a difference in the friction coefficients of the sheets due to the different qualities of materials of the sheets and the like.
An example of a sheet transport apparatus that addresses the above-described problem is described in Japanese Patent Application Laid-Open No. H05-319630. FIG. 9 is a schematic drawing of a sheet transport apparatus described in Japanese Patent Application Laid-Open No. H05-319630. In a sheet transport apparatus 1, a sheet is transported to paired rollers 7 and is temporarily stopped with the sheet inserted between the paired rollers 7. After a predetermined time elapses from the start of discharge operation of the sheet, the transport of the sheet is again started from the position of the paired rollers 7. Thus, if the sheet is delayed due to the slippage of a pickup roller 2 corresponding to the above-described roller, the conventional sheet transport apparatus 1 recovers the delay, thereby reducing variations in transport.
Also, the conventional sheet transport apparatus 1 is incorporated in an image forming apparatus that forms an image on a sheet in an image forming portion 15. In the sheet transport apparatus 1, therefore, on the assumption that any slippage occurs in the pickup roller 2 when the sheet is discharged, the sheet is discharged at a higher sheet transport speed V2 than a sheet transport speed V1 of the image forming portion 15. The sheet is temporarily stopped at the paired rollers 7 and after the restart the sheet is discharged at the same sheet transport speed as the sheet transport speed V1 of the image forming portion 15. In this way, the sheet transport apparatus 1 prevents the image forming efficiency of the image forming apparatus from decreasing.
In addition, in order to detect the position of the sheet when the sheet is temporarily stopped at the paired rollers 7 that are in a predetermined position, an elapsed time after the sheet passes a sensor 9 serving as a reference is measured using a timer and a stop position (=speed×time) is calculated in a calculation portion. When a control portion determines that the sheet is transported to the stop position, the control portion causes the paired rollers 7 to stop rotating with the sheet inserted between the paired rollers 7. That is, the sheet stops when the leading end of the sheet protrudes from the paired rollers 7 to the downstream side of the sheet transport direction.
If there are no slippage of the pickup roller 2 and no delay of the sheet, however, the conventional sheet transport apparatus 1 then transports the sheet to the paired rollers 7 at an earlier timing, causes the sheet to stand by at that position for a predetermined time, and again transport the sheet. The stop time is thus extended by the early arrival time of the sheet at the paired rollers 7, resulting in a lower sheet transport efficiency.
Accordingly, the conventional sheet transport apparatus transports the sheet to the paired rollers 7 at a higher speed to increase the sheet transport efficiency. This results in, however, that vibrations of the machinery that transports the sheet and a motor become greater, which causes noise.
Also, an image forming apparatus having a sheet transport apparatus with great vibrations of the machinery that transports the sheet and the motor may possibly be affected by the vibrations, thereby degrading the quality of an image formed on the sheet.
Also, since, in the conventional sheet transport apparatus, the elapsed time after the sheet passes the sensor 9 serving as a reference is measured using a timer to calculate a stop position of the sheet, the assumed speed and time are uneven after the sensor is activated due to the slippage of the paired rollers 7, an error of the timer that measures time, an error due to the delay of software processing and the like. The stop position of the sheet that is inserted in the paired rollers 7 and stopped is thus uneven. That is, the length to which the sheet protrudes from the paired rollers 7 to the downstream side is uneven. The conventional sheet transport apparatus cannot therefore sometimes transport the sheets with regular intervals when the sheets are continuously transported.
It is an object of the present invention is to provide a sheet transport apparatus that transports a sheet to a predetermined position at a proper timing.
It is an object of the present invention is to provide an image forming apparatus having a sheet transport apparatus with reduced vibrations and an improved quality of an image formed on a sheet.